### 29.1.1 Contact interaction analysis: overview

This section presents an overview of the contact analysis capabilities in ABAQUS. The contact modeling capabilities available in ABAQUS/Standard and ABAQUS/Explicit differ significantly; therefore, they are discussed separately. A comparison of the capabilities is provided at the end of this section.

### Contact simulation capabilities in ABAQUS/Standard

There are two methods for modeling contact interactions in ABAQUS/Standard: using surfaces or using contact elements.

#### Surface-based contact simulations

Most contact problems are modeled by using surface-based contact. The following types of problems can be simulated with surface-based contact:

• Contact between two deformable bodies. The structures can be either two- or three-dimensional, and they can undergo either small or finite sliding. Examples of such problems include the assembly of a cylinder head gasket and the slipping between the two components of a threaded connector.

• Contact between a rigid surface and a deformable body. The structures can be either two- or three-dimensional, and they can undergo either small or finite sliding. Examples of such problems include metal forming simulations and analyses of rubber seals being compressed between two components.

• Finite-sliding self-contact of a single deformable body. An example of such a problem is a complex rubber seal that folds over on itself.

• Small-sliding or finite-sliding interaction between a set of points and a rigid surface. These models can be either two- or three-dimensional. An example of this type of problem is the pull-in of an underwater cable that is resting on the seabed, with the seabed modeled as a rigid surface.

• Contact between a set of points and a deformable surface. These models can be either two- or three-dimensional. An example of this class of contact problem is the design of a bearing where one of the bearing surfaces is modeled with substructures.

• Problems where two separate surfaces need to be “tied” together so that there is no relative motion between them. This modeling technique allows for joining dissimilar meshes.

• Coupled thermal-mechanical interaction between deformable bodies with finite relative motion. The analysis of a disc brake is an example of such a problem.

• Coupled pore fluid-mechanical interaction between bodies. An example of this type of problem is the analysis of the interfaces between layered soil material at a waste disposal site.

Coupled thermal-mechanical interactions can be included in any of the above examples as long as both of the surfaces are deformable.

There are three steps in defining a surface-based contact simulation in ABAQUS/Standard:

• defining the surfaces of the bodies that could potentially be in contact;

• specifying which surfaces interact with one another; and

• defining the mechanical and thermal property models that govern the behavior of the surfaces when they are in contact.

##### Defining surfaces

Surfaces are considered part of the model definition, so all surfaces that may be needed in an analysis must be defined at the beginning of the simulation.

ABAQUS has three classifications of contact surfaces:

The restrictions on surfaces created in ABAQUS are discussed in Surfaces: overview, Section 2.3.1.

##### Defining contact between surfaces

Once surfaces have been created, you must specify which pairs of surfaces can interact with each other during the analysis. At least one surface of the pair must be a non-node-based surface. The definition of these contact pairs is discussed in detail in Defining contact pairs in ABAQUS/Standard, Section 29.2.1.

##### Defining property models for contact simulations

Some of the mechanical contact property models available in ABAQUS/Standard include:

Surface interaction in thermal or coupled thermal-mechanical contact simulations can include heat exchange by conduction and radiation as well as the generation of frictional heat in coupled simulations. These contact property models are discussed in Thermal contact properties, Section 30.2.1.

Surface interaction in coupled thermal-electrical problems includes flow of electrical current between the surfaces in addition to the thermal property models mentioned previously. The thermal-electrical property model is discussed in Electrical contact properties, Section 30.3.1.

The contact property model for pore fluid simulations is discussed in Pore fluid contact properties, Section 30.4.1. The model includes pore fluid flow that is both normal and tangential to the surfaces.

#### Contact simulations requiring contact elements

The surface-based contact method cannot be used for certain classes of problems. ABAQUS/Standard provides a library of contact elements for these problems. Examples of such problems are:

##### Defining a contact simulation using contact elements

The steps required for defining a contact simulation using contact elements are similar to those needed when defining a surface-based contact simulation:

• create the contact elements or slide lines;

• assign element section properties to the contact elements;

• associate sets of contact elements with the slide lines if applicable; and

• define the contact property models for the contact elements.

The first three steps are discussed in Chapter 31, Contact Elements in ABAQUS/Standard,” in the sections for each type of contact element. The contact property models for contact elements are identical to those used for surface-based contact.

### Contact simulation capabilities in ABAQUS/Explicit

ABAQUS/Explicit provides two algorithms for modeling contact interactions. The general (“automatic”) contact algorithm allows very simple definitions of contact with very few restrictions on the types of surfaces involved (see Defining general contact in ABAQUS/Explicit, Section 29.3). The contact pair algorithm has more restrictions on the types of surfaces involved and often requires more careful definition of contact; however, it allows for some interaction behaviors that currently are not available with the general contact algorithm (see Defining contact pairs in ABAQUS/Explicit, Section 29.4).

The two contact algorithms combine to provide the following capabilities in ABAQUS/Explicit:

• Contact between rigid and/or deformable bodies.

• Contact of a body with itself.

• Finite-sliding or small-sliding contact.

• Contact with eroding bodies (due to element failure). A node-based surface must be used to model the eroding body if contact pairs are used. General contact allows element-based surfaces to be defined on eroding bodies, so contact between any number of eroding bodies can be modeled.

• General constitutive models for the contact behavior, relating constraint pressure and shear traction to penetration distance and relative tangential motion.

• Thermal interaction at the surface of a body; for example, conductive heat transfer.

#### Choosing the contact algorithm

Contact definitions are not entirely automatic with the general contact algorithm but are greatly simplified. The generality of this algorithm is primarily in the relaxed restrictions on the surfaces that can be used in contact. The general contact algorithm allows the following (none of which are allowed with the contact pair algorithm):

• A surface can span unattached bodies.

• More than two surface facets can share a common edge (allowing “T-intersections” in shells, for example).

• A surface can include deformable and rigid regions; furthermore, the rigid regions need not be from the same rigid body.

• A surface can have mixed parent element types; for example, adjacent surface facets can be on shell and solid elements.

• A surface can be based on combinations of surfaces of the same type.

• An element-based surface can be defined on the interior of solid bodies for use in modeling erosion due to element failure.

Other benefits of the general contact algorithm include the following:
• The general contact algorithm can enforce edge-to-edge contact for geometric feature edges, perimeter edges of structural elements, and edges defined by beam and truss elements, unlike the contact pair algorithm.

• The general contact algorithm eliminates problematic, nonphysical “bull-nose” extensions that may arise at shell surface perimeters in the contact pair algorithm.

• With the general contact algorithm each slave node can see contact with multiple facets per increment; with the contact pair algorithm each slave node can see contact with only one facet per increment unless multiple surface pairings are specified. Likewise, each contact edge can see contact with multiple edges per increment when the general contact algorithm is used.

• The general contact algorithm has some built-in smoothing for element-based surfaces that can be beneficial for modeling contact near corners.

• The general contact algorithm, unlike the contact pair algorithm, removes contact faces and contact edges from the contact domain and, if an interior surface is defined, activates newly exposed surface faces as elements fail. Thus, element-based surfaces can be used to describe eroding solids. This allows contact between multiple eroding solids to be modeled since a node-based surface does not need to be defined on the eroding solid.

• Contact state information (such as the proper contact normal orientation for double-sided surfaces) is transferred across step boundaries in the general contact algorithm even if the contact domain is modified; in the contact pair algorithm, contact state information is transferred across step boundaries only for contact pairs with no modifications.

• The contact interaction domain, contact properties, and surface attributes are specified independently for the general contact algorithm, offering a more flexible way to add detail incrementally to a model.

• The general contact algorithm does not place any restrictions on the domain decomposition for domain level parallelization (see Parallel execution in ABAQUS/Explicit, Section 11.9.3).

• The general contact algorithm has been developed to minimize the need for algorithmic controls.

See Knee bolster impact with general contact, Section 2.1.9 of the ABAQUS Example Problems Manual; Crimp forming with general contact, Section 2.1.10 of the ABAQUS Example Problems Manual; and Collapse of a stack of blocks with general contact, Section 2.1.11 of the ABAQUS Example Problems Manual, for example analyses that use the general contact algorithm.

Although the general contact algorithm is more powerful and allows for simpler contact definitions, the contact pair algorithm must be used in certain cases where more specialized contact features are desired. The following features are available only when the contact pair algorithm is used:

In addition, the general contact algorithm places more restrictions on adaptive meshing than the contact pair algorithm (see Defining ALE adaptive mesh domains in ABAQUS/Explicit, Section 12.2.2). The choice of contact algorithm may affect the speedup factor if loop-level parallelization is used: the contact pair algorithm includes some loop-level parallelization, while the general contact algorithm has no loop-level parallelization. Contact output is more complete for a contact pair analysis.

The two contact algorithms can be used together in the same ABAQUS/Explicit analysis. The general contact algorithm automatically avoids processing interactions that are treated by the contact pair algorithm.

#### Defining a contact simulation

A contact simulation using either algorithm in ABAQUS/Explicit is defined by specifying:

• surface definitions for the bodies that could potentially be in contact;

• the surfaces that interact with one another (the contact interactions);

• any nondefault surface properties to be considered in the contact interactions;

• the mechanical and thermal contact property models, such as the pressure-overclosure relationship or the contact conduction coefficient;

• any nondefault aspects of the contact formulation; and

• any algorithmic contact controls for the analysis.

In many cases you will need to specify only which surfaces interact, because the default settings for the other aspects of a contact simulation are often appropriate. The most common exception is specification of a friction coefficient; by default, friction is not modeled.

##### Surfaces

Surfaces can be defined at the beginning of a simulation or upon restart as part of the model definition (see Surfaces: overview, Section 2.3.1). ABAQUS has three classifications of contact surfaces:

Surfaces of the same type can be combined to create new surfaces (see Operating on surfaces, Section 2.3.5). However, with regard to contact a combined surface can be used only with general contact.

When the general contact algorithm is used, ABAQUS/Explicit also provides a default all-inclusive, automatically defined surface that includes all element-based surface facets as well as all analytical rigid surfaces in the model.

##### Contact interactions

Contact interactions for both contact algorithms are defined by specifying surface pairings and self-contact surfaces. General contact interactions typically are defined by specifying self-contact for the default surface, which allows an easy, yet powerful, definition of contact. (Self-contact for a surface that spans multiple bodies implies self-contact for each body as well as contact between the bodies.)

At least one surface in an interaction must be a non-node-based surface, and at least one surface in an interaction must be a non-analytical rigid surface.

The definition of general contact interactions, including further restrictions on the surfaces that can be used in them, is discussed in detail in Defining general contact interactions, Section 29.3.1. The definition of contact pairs, including further restrictions on the surfaces that can be used in them, is discussed in detail in Defining contact pairs in ABAQUS/Explicit, Section 29.4.1.

##### Surface properties

Nondefault surface properties (such as thickness and, in some cases, offset) can be defined for particular surfaces in a contact model. In addition, you can control which edges of a surface will be included in the general contact domain. The general contact algorithm uses the surface property assignments specified for contact purposes (see Surface properties for general contact, Section 29.3.2); the contact pair algorithm uses the surface properties specified in the surface definition (see Surface properties for ABAQUS/Explicit contact pairs, Section 29.4.2).

##### Contact properties

Contact interactions in a model can refer to a contact property definition, in much the same way that elements refer to an element property definition. By default, the surfaces interact (have constraints) only in the normal direction to resist penetration. The other mechanical contact interaction models available in ABAQUS/Explicit depend on the contact algorithm used (see Mechanical contact properties: overview, Section 30.1.1). Some of the available models are:

The thermal surface interaction models available in ABAQUS/Explicit (for the contact pair algorithm only) are discussed in Thermal contact properties, Section 30.2.1.

Contact interaction models are defined as model data for general contact analyses and as history data for contact pair analyses. Information on assigning contact properties to specific contact interactions can be found in Contact properties for general contact, Section 29.3.3, and Contact properties for ABAQUS/Explicit contact pairs, Section 29.4.3.

##### Contact formulation

The contact formulation includes the constraint enforcement method, the contact surface weighting, and the sliding formulation. Nondefault aspects of the contact formulation can be specified for particular interactions in a contact model, depending on the contact algorithm chosen. See Contact formulation for general contact, Section 29.3.4, for details on the formulation used with general contact. See Contact formulation for ABAQUS/Explicit contact pairs, Section 29.4.4, for details on the formulation used with the contact pair algorithm.

##### Algorithmic contact controls

The default algorithmic controls for contact analyses are usually sufficient, but additional solution controls are available for some special cases. The available solution controls depend on the contact algorithm used. See Contact controls for general contact, Section 29.3.6, for information on nondefault algorithmic controls for general contact. See Defining contact pairs in ABAQUS/Explicit, Section 29.4.1, and Common difficulties associated with contact modeling using the contact pair algorithm in ABAQUS/Explicit, Section 29.4.6, for information on nondefault algorithmic controls for the contact pair algorithm.

### Compatibility between ABAQUS/Standard and ABAQUS/Explicit

There are fundamental differences in the mechanical contact algorithms in ABAQUS/Standard and ABAQUS/Explicit, even though the input syntax for ABAQUS/Standard and the contact pair algorithm in ABAQUS/Explicit are similar. These differences are reflected in how and where contact conditions are defined in the input file. The main differences are the following:

As a result of these differences, contact definitions specified in an ABAQUS/Standard analysis cannot be imported into an ABAQUS/Explicit analysis and vice versa (see Transferring results between ABAQUS/Explicit and ABAQUS/Standard, Section 9.2.2).